Surface-supported gold cages

Going for Gold(-Standard): Attaining Coupled Cluster Accuracy in Oxide-Supported Nanoclusters

The structure of oxide-supported metal nanoclusters plays an essential role in their sharply enhanced catalytic activity over that of bulk metals. Simulations provide the atomic-scale resolution needed to understand these systems. However, the sensitive mix of metal-metal and metal-support interactions, which govern their structure, puts stringent requirements on the method used, requiring calculations beyond standard density functional theory (DFT). The method of choice is coupled cluster theory [specifically CCSD(T)], but its computational cost has so far prevented its application to these systems. In this work, we showcase two approaches to make CCSD(T) accuracy readily achievable in oxide-supported nanoclusters. First, we leverage the SKZCAM protocol to provide the first benchmarks of oxide-supported nanoclusters, revealing that it is specifically metal-metal interactions that are challenging to capture with DFT. Second, we propose a CCSD(T) correction (ΔCC) to the metal-metal interaction errors in DFT, reaching accuracy comparable to that of the SKZCAM protocol at significantly lower cost. This approach forges a path toward studying larger systems at reliable accuracy, which we highlight by identifying a ground-state structure in agreement with experiments for Au20 on MgO, a challenging system where DFT models have yielded conflicting predictions.

Tempering of Au nanoclusters: capturing the temperature-dependent competition among structural motifs

A computational approach to determine the equilibrium structures of nanoclusters in the whole temperature range from 0 K to melting is developed. Our approach relies on Parallel Tempering Molecular Dynamics (PTMD) simulations complemented by Harmonic Superposition Approximation (HSA) calculations and global optimization searches, thus combining the accuracy of global optimization and HSA in describing the low-energy part of configuration space, together with the PTMD thorough sampling of high-energy configurations. This combined methodology is shown to be instrumental towards revealing the temperature-dependent structural motifs in Au nanoclusters of sizes 90, 147, and 201 atoms. The reported phenomenology is particularly rich, displaying a size- and temperature-dependent competition between the global energy minimum and other structural motifs. In the case of Au₉₀ and Au₁₄₇, the global minimum is also the dominant structure at finite temperatures. In contrast, the Au₂₀₁ cluster undergoes a solid-solid transformation at low temperature (<200 K). Results indicate that PTMD and HSA very well agree at intermediate temperatures, between 300 and 400 K. For higher temperatures, PTMD gives an accurate description of equilibrium, while HSA fails in describing the melting range. On the other hand, HSA is more efficient in catching low-temperature structural transitions. Finally, we describe the elusive structures close to the melting region which can present complex and defective geometries, that are otherwise difficult to characterize through experimental imaging.

Exploring the materials space in the smallest particle size range: From heterogeneous catalysis to electrocatalysis and photocatalysis

Ultrasmall clusters of subnanometer size can possess unique and even unexpected physical and chemical propensities which make them interesting in various fields of basic science and for potential applications, such...

Melting of large Pt@MgO(1 0 0) icosahedra

On the basis of ab initio calculations, we present a new parametrisation of the Vervisch-Mottet-Goniakowski (VMG) potential (Vervisch et al 2002 Phys. Rev. B 24 245411) for modelling the oxide-metal interaction. Applying this model to mimic the finite temperature behaviour of large platinum icosahedra deposited on the pristine MgO(1 0 0), we find the nanoparticle undergoes two solid-solid transitions. At 650 K the 'squarisation' of the interface layer, while a full reshaping towards a fcc architecture takes place above 950 K. In between, a quite long-lived intermediate state with a (1 0 0) interface but with an icosahedral cap is observed. Our approach reproduces experimental observations, including wetting behaviour and the lack of surface diffusion.

Formation and Stability of Low-Dimensional Structures for Group VIIIB and IB Transition Metals: The Role of sd4 Hybridization

A quasi-sd⁴ hybridization state for group VIIIB and IB face-centered cubic (FCC) transition metals in low-dimensional nanostructures is identified, in contrast to the sd⁵ hybridization state in bulk. For Au, a novel three-shelled nanowire is designed with a hexagonal close-packed core in the sd⁵ hybridization, wrapped by FCC-(111) shell that adopts the quasi-sd⁴ hybridization. This new nanostructure exhibits remarkable stability and electronic properties.

Raman modes, dipole moment and chirality in periodically positioned Au8clusters

The solution-based assembly of h-Au₈clusters into a local periodic structure (LPS) of periodicity 1.47 nm with negative chirality as confirmed by experimental and theoretical analyses exhibits excitonic couplet state.

Doped golden fullerene cages

A first-principles investigation of the effect of the doping of golden cages of 32 atoms is proposed.

Atomic Details of Interfacial Interaction in Gold Nanoparticles Supported on MgO(001)

Atomic-scale imaging using aberration-corrected scanning transmission electron microscopy reveals direct evidence for semicoherent interfacial epitaxy and coordinate-dependent surface contraction for the fcc (001) oriented Au nanoparticles (2-3 nm in diameter), suggesting that their interaction with the substrate is weaker than previously assumed. A significant change in interfacial separation distance from 2.47 ± 0.12 Å for the fcc (001) oriented Au nanoparticles to 3.07 ± 0.11 Å for the fcc (111) oriented Au nanoparticles has also been observed. These results are used to verify the atomistic models generated by the global optimization calculations, which shed further light on the intricate relation between the interfacial energy and the atomic structure of the nanoparticle and their combined effect on the inhomogeneous surface structural relaxation of supported nanoparticles.

Determination of the structures of small gold clusters on stepped magnesia by density functional calculations

The structural modifications of small supported gold clusters caused by realistic surface defects (steps) in the MgO(001) support are investigated by computational methods. The most stable gold cluster structures on a stepped MgO(001) surface are searched for in the size range up to 24 Au atoms, and locally optimized by density-functional calculations. Several structural motifs are found within energy differences of 1 eV: inclined leaflets, arched leaflets, pyramidal hollow cages and compact structures. We show that the interaction with the step clearly modifies the structures with respect to adsorption on the flat defect-free surface. We find that leaflet structures clearly dominate for smaller sizes. These leaflets are either inclined and quasi-horizontal, or arched, at variance with the case of the flat surface in which vertical leaflets prevail. With increasing cluster size pyramidal hollow cages begin to compete against leaflet structures. Cage structures become more and more favourable as size increases. The only exception is size 20, at which the tetrahedron is found as the most stable isomer. This tetrahedron is however quite distorted. The comparison of two different exchange-correlation functionals (Perdew-Burke-Ernzerhof and local density approximation) show the same qualitative trends.

Hollow polyhedral structures in small gold-sulfide clusters

Using ab initio methods, we investigate the structural evolution of a family of gold-sulfide cluster anions (Au(m)S(n)(-)). We show that this family of clusters exhibits simple size-evolution rules and novel hollow polyhedron structures. The highly stable Au(m)S(n)(-) species such as Au(6)S(4)(-), Au(9)S(5)(-), Au(9)S(6)(-), Au(10)S(6)(-), Au(11)S(6)(-), Au(12)S(8)(-), and Au(13)S(8)(-) detected in the recent ion mobility mass spectrometry experiment of Au(25)(SCH(2)CH(2)Ph)(18) (Angel et al. ACS Nano2010, 4, 4691) are found to possess either quasi-tetrahedron, pyramidal, quasi-triangular prism, or quasi-cuboctahedron structures. The formation of these polyhedron structures are attributed to the high stability of the S-Au-S structural unit. A unique "edge-to-face" growth mechanism is proposed to understand the structural evolution of the small Au(m)S(n)(-) cluster. A 3:2 ratio rule of Au/S is suggested for the formation of a hollow polyhedron structure among small-sized Au(m)S(n) clusters.

Multishell intermetallic onions by symmetrical configuration of ordered domains

Ordered domains are utilized to construct new nanostructures, i.e., multishell intermetallic onions, which are formed by symmetrical configuration of ordered domains. Through density-functional theory calculations, we have shown that the energy penalties for introducing antiphase boundaries into the nanoparticles are small in some alloy systems compared to typical surface energies, making it feasible to prepare intermetallic onions by tuning surface energies. The unique surface atomic arrangements would provide opportunities for developing novel materials like efficient catalysts.

Prediction of the structures of free and oxide-supported nanoparticles by means of atomistic approaches: the benchmark case of nickel clusters

The structures of Ni/MgO nanoparticles are studied by means of global optimization searches. The results from four different model potentials, sharing the same functional forms but different parametrizations, are reported and compared. Two parametrizations over four give qualitatively correct results, and one of them is also quantitatively satisfactory. The other models fail to explain some qualitative features observed in the experiments, such as the formation of hcp nanodots at small sizes or the transition to fcc structures at large sizes. The important features that an atomistic potential must present for the correct prediction of Ni cluster structures are discussed and generalized.